Numerical solutions are computed within MOM by dividing the ocean volume into a three dimensional lattice, discretizing the equations within each lattice cell, and solving these equations by finite difference techniques. The solution method could be formulated in a straightforward manner, but the result would be a numerically inefficient algorithm. Instead, Bryan (1969) introduced a fundamental technique to ocean modeling in which the ocean velocity field is separated into its depth averaged part and the deviation from the depth average. The following discussion introduces the motivations and ideas behind this approach.